The present invention concerns an improved process for the manufacture of insulated winding wires through extrusion of thermoplastics.
Lacquer-insulated winding wires, so-called "enamelled wire", are accurately characterized in the German norm DIN 46435 of April 1977. They appear to a great extent in electrical machine construction, transformer construction and in electronics.
The metal wire, preferably of copper or of aluminum, is insulated with a thin, yet extremely mechanically and thermally resistant synthetic resin enamel coat.
The manufacture of such enamelled wire is performed with wire lacquering machines by several continuous applications of a wire lacquer onto the metal wire. With regard to the noxiousness of the solvent for the wire lacquer which hence becomes an environmental problem, wire lacquer dispersions and aqueous solutions of wire enamel resins as well as fused resin are employed.
All of the known methods are in consideration of the comparatively inferior rate of drawing off therewith attainable very time- and work-consuming.
In the cable industry the extrusion of thermoplastics for thick-wall layers of electrical conductor bundles as well as for the manufacture of conducting wires has been known for some time.
In the older applications (see, e.g. German Allowed Application No. 26 38 763, corresponding to U.S. Pat. No. 4,145,474, which is hereby incorporated by reference) a method for producing lacquer-insulated winding wires by extrusion of thermoplastics is described.
This accomplished, by use of the applicants' executed older application therewith, a decided contribution to the overcoming of the prejudice that the attainment of thinner insulating layers, such as those required by DIN 46435, was not possible with an extrusion method. According to German Allowed Application No. 26 38 763, part crystalline thermoplastic polycondensates with crystallite melting points above 170.degree. C., preferably above 250.degree. C., can be used as thermoplastic material for the extrusion coating of winding wires.
A disadvantage of the part crystalline polycondensates according to German Allowed Application 26 38 763, particularly of polyethylene terephthalate according to Example 1, is--as it was recently found--the tendency of the thermoplastic coating to form cracks.
After a storage time of from a few days to several weeks and preferably after the rewinding of the coated wires there formed concentrically running cracks which were very fine on the surface and which are believed to be connected with the crystallization and shrinking processes of the polymers.
It can be assumed that these cracks, even when they do not penetrate to the metal surface, do represent an interference with some of the properties of the coil wires.
It was therefore surprising to discover an improved process for the manufacture of winding wires through extrusion of thermoplastics, which overcame the disadvantages described.